The cluster-stats API provides similar output to the node-stats. There
is one crucial difference: Node Stats shows you statistics per node, while
cluster-stats shows you the sum total of all nodes in a single metric.

This provides some useful stats to glance at. You can see for example, that your entire cluster
is using 50% of the available heap or that filter cache is not evicting heavily. Its
main use is to provide a quick summary that is more extensive than
the cluster-health, but less detailed than node-stats. It is also useful for
clusters that are very large, which makes node-stats output difficult
to read.